The present invention relates to a photomultiplier tube designed to detect through multiplication weak light radiated onto a faceplate, and a manufacturing method thereof. The present invention also relates to a radiation detector using this type of photomultiplier tube.
Photomultiplier tubes are produced at various sizes to suit different uses. Conventional photomultiplier tubes are generally configured of a hermetically sealed vessel constructed entirely of glass. This type of photomultiplier tube has a substantially cylindrical glass side tube with a planar and substantially circular stem fitted on the bottom open end. A gas burner is applied to the area in which the periphery of the stem contacts the inner surface of the side tube, and the two are fused together by heat to form an airtight seal. A planar and substantially circular transparent faceplate is fused in the same manner to the top opening of the side tube, forming an airtight seal. A glass exhaust tube is provided in the stem, forming a fluid connection with the inside of the hermetically sealed vessel. After evacuating the vessel through the exhaust tube, an alkaline metal vapor is introduced into the vessel to form a photocathode on the inner surface of the faceplate and also to activate the secondary electron emitting surface of the electron multiplying section disposed inside the vessel.
As described above, the side tube and stem are fused together with heat from a gas burner in order to construct a photomultiplier tube having an airtight glass vessel. Further, after introducing the alkaline metal vapor, the exhaust tube is pinched off and sealed with the gas burner. Accordingly, it is necessary to prevent heat from damaging the electron multiplying section. Therefore, it has been necessary to maintain a distance of about 20-30 mm from the airtight fused junctions to the bottom of the electron multiplying section. Unfortunately, this inhibits downsizing of the photomultiplier tube.
While it depends on the application, the advantages of downsizing a photomultiplier tube are generally great. One example is a sanitation monitor that is used for detecting bacteria in restaurants and the like. The sanitation monitor employs a photomultiplier tube to detect light that is emitted from reactions between drugs and bacteria. Since this type of monitoring device should be portable and easy to use, the photomultiplier tubes used in the monitor must be small. A small enough photomultiplier tube can be mounted directly on a circuit board and treated in the same way as a resistor or capacitor, thereby making the tubes more convenient for device construction.
In light of these demands, small photomultiplier tubes having metal side tubes have been developed for practical uses in recent years. An example of this type of photomultiplier tube described in Japanese Unexamined Patent Application Publication Nos. HEI-5-290793 and HEI-9-306416 has a metal side tube having a polygonal cross-section with a flange portion protruding laterally from the bottom of the tube. Similarly, the metal stem plate is provided with a flange portion that protrudes laterally. The flange portions of the side tube and stem plate overlap and are fused through resistance welding to form a hermetically sealed vessel. Resistance welding is performed in such a way that current is applied to the joining parts, which are heated through the heat generated from resistance. When the parts reach an appropriate temperature, pressure is applied to weld them together. Hence, it is possible to avoid the thermal affects that resistance welding has on the electron multiplying section. As a result, the distance between the stem plate and electron multiplying section can be reduced, thereby achieving a smaller photomultiplier tube that is shorter in the axial direction.
With a photomultiplier tube of this construction, however, the flange portions required for resistance welding interfere with the use of the tube. For example, when photomultiplier tubes are used in gamma cameras and the like, in particular, it is necessary to arrange a plurality of photomultiplier tubes closely together and form a large area for receiving light. With this configuration, the flange portions contact other flange portions, forming dead spaces. Dead spaces are a hindrance to achieving a high-performance detecting device.
The present invention has been made to solve the above-described problems, and accordingly it is an object of the present invention to provide a photomultiplier tube and a method for manufacturing a smaller multiplier tube. It is another object of the present invention to provide a radiation detector with improved performance.
These objects will be attained by a method of manufacturing a photomultiplier tube having a faceplate, a photocathode for emitting electrons in response to light incident on the faceplate, an electron multiplying section for multiplying the electrons emitted from the photocathode, an anode for outputting an output signal based on the electrons multiplied by the electron multiplying section, a stem plate for fixedly supporting the electron multiplying section and the anode with stem pins, and a side tube with the stem plate fixed on one open end and the faceplate fixed on the other open end and enclosing the electron multiplying section and the anode, the method characterized by the steps of: providing a metal side tube formed of metal and a stem plate such that at least a portion contacting the metal side tube is formed of metal; aligning the metal side tube with the stem plate so that an outer edge of the stem plate does not protrude further externally than an outer surface of the metal side tube; and fusing the metal side tube to the stem plate at a point of contact between the metal side tube and the stem plate by laser welding or electron beam welding to form an airtight vessel.
The side tube is engaged with the stem plate such that only the outer side surface of the side tube or both the outer surface of the side tube and at least a portion of the outermost side edge of the stem plate are exposed on the outer surface of the airtight vessel, the parts being reshaped as necessary.
According to another aspect of the present invention, there is provided a photomultiplier tube including a faceplate, a photocathode for emitting electrons in response to light incident on the faceplate, an electron multiplying section, disposed inside an airtight vessel, for multiplying the electrons emitted from the photocathode, and an anode for outputting an output signal based on the electrons multiplied by the electron multiplying section, characterized in that the airtight vessel comprises:
a stem plate for fixedly supporting the electron multiplying section and the anode with stem pins;
a metal side tube with the stem plate fixed on one open end, and enclosing the electron multiplying section and the anode; and
a faceplate fixed on the other open end of the metal side tube,
wherein the stem plate is welded on the one open end of the metal side tube, a top surface of the stem plate contacting a bottom end of the metal side tube such that an outer surface of the metal side tube is flush with an edge surface of the stem plate, at least a portion of the top surface of the stem plate in contact with the metal side tube being formed of metal.
Welding the side tube to the stem plate such that the outer surface of the side tube is flush with the edge surface of the stem plate eliminates the flange-like protrusions on the bottom of the photomultiplier tube. While posing difficulties for resistance welding, this construction allows the external dimensions of the photomultiplier tube to be reduced and enables the side tubes to be positioned adjacent one another when justaposing a plurality of photomultiplier tubes. Accordingly, employing a welding method to join the metal stem plate and side tube enables the photomultiplier tubes to be more densely packed together.
A cutout portion may be provided in the top surface on the edge of the stem plate for supporting the bottom end of the side tube. Hence, the side tube can be placed stably on the stem plate and easily position the side tube in relation to the stem plate before welding the two together. Moreover, the reinforced construction can oppose a force toward the inside of the vessel 5A attempting to bend the side tube 2.
It is preferable that the side tube be fusion welded to the stem plate. In contrast to resistance welding, a fusion welding method does not require that pressure be added to the portions of the side tube and stem plate that are being joined. Accordingly, residual stress that can lead to cracking during operations is not generated at this junction, thereby greatly improving durability of the apparatus.
As the fusion welding, laser welding or electron beam welding is preferable. The methods of laser welding and electron beam welding generate little heat at the junction. Therefore, the glass tablets fixing the stem pins to the stem plate are not apt to crack from the effects of the heat when the stem pins are near the side tube. Accordingly, it is possible to move these stem pins closer to the side tube and expand the electron multiplying section laterally, creating a greater electron receiving surface area in the electron multiplying section.
The entire stem plate can be formed of metal, or a metal stem support member in contact with the lower end of the side tube extending substantially in the axial direction of the same, and a glass stem plate.
According to another aspect of the present invention, there is provided a radiation detector including a scintillator for emitting fluorescent light in response to radiation generated from an object of analysis, a plurality of photomultiplier tubes, each having a faceplate disposed in opposition to the scintillator, for outputting electric charges based on fluorescent light emitted from the scintillator, and a position calculating section for performing calculations on the electric charges output from the plurality of photomultiplier tubes and outputting positioning signals of radiation issued in the object of analysis. The photomultiplier tube used in this radiation detector includes a photocathode for emitting electrons in response to light incident on the faceplate; an electron multiplying section, disposed inside an airtight vessel, for multiplying the electrons emitted from the photocathode; and an anode for outputting an output signal based on the electrons multiplied by the electron multiplying section. The airtight vessel comprises:
a metal stem plate for fixedly supporting the electron multiplying section and the anode with stem pins;
a metal side tube with the metal stem plate fixed on one open end, and enclosing the electron multiplying section and the anode, wherein the metal stem plate is fixed by welding to the metal side tube such that an outermost edge of the metal stem plate does not protrude outward from an outer surface of the metal side tube: and
the faceplate fixed on the other open end of the metal side tube.
Joining the side tube and stem plate through a welding process in the photomultiplier tubes used in this type of radiation detector, such that the outer surface of the metal side tube is flush with the edge surface of the stem plate, eliminates the flange-like protrusion from the bottom of the photomultiplier tube. Accordingly, although resistance welding is not appropriate for this construction, the dimensions of the photomultiplier tube can be reduced and the side tubes can be arranged in close contact with one another when employing a plurality of juxtaposed photomultiplier tubes. Accordingly, when the photomultiplier tubes are arranged in order that the faceplates confront the scintillator, the tubes can be densely packed, thereby easily securing a light receiving area with an extremely small amount of dead space, which forms non-sensitive areas. This configuration contributes to further improved performance of the radiation detector.